This invention is directed to stable sizing compositions, wherein the film-former is a polyester resin with free carboxyl side and/or end groups; and to a method of making said sizing composition in order to permit more efficient use of the sizing composition in treating glass fibers.
Typically glass fiber strands are produced by combining a multitude of fine glass filaments that are formed by the rapid attenuation from molten cones of glass at the tips of small orifices in a bushing. An example of such an operation is shown in U.S. Pat. No. 2,133,238. In order to protect the glass filaments from interfilament abrasion, the filaments are coated during their formation with a sizing composition which contains a lubricant to protect the glass fiber strand against handling equipment during further processing. The size also contains a binder to give the glass fiber strand integrity and workability for any standard textile or reinforcement use. In addition the size contains a coupling agent to improve the interfacial bond between the glass and matrix resin that the glass is to reinforce. After the filaments have been treated with the size composition and gathered together into a strand the glass fiber strand is wound around a rapidly rotating drum to form a package.
Over the years the development of sizes has progressed so that the use of glass fibers to reinforce thermoplastic and thermosetting polymers is well accepted. The usefulness of glass fibers as a reinforcement for plastics relies on the efficient transfer of stresses from the relatively weak polymers constituting the matrix to the much stronger glass fibers. For an efficient transfer of stresses to occur there must be a good adhesion between the matrix and the surface of the glass fiber reinforcement. In order to obtain good adhesion between the matrix and the surface of the glass, the sizing composition must overcome the hydrophobic tendencies of the polymer sizing and the hydrophilic tendencies of the glass surface.
To accomplish the objectives of providing the desired handling characteristics of the glass fiber strands and performance characteristics for adapting glass fibers as reinforcement for polymeric materials, sizing compositions contain several components. These include a film former that is soluble and compatible with the polymeric material to be reinforced, lubricant, coupling agent, and additives.
Coupling agents that have been used include Werner complex compounds and organo-silicon compounds. These compounds coordinate or react with groupings on the glass fiber surface and the polymeric material by a tie-in type bond. The unsaturated organo-silicon compounds such as silanes or polysiloxanes accomplish this result because the silicon atom has attached to it one or more groups which can react with the glass surface resulting in the removal of hydroxyl groups and one or more groups which bonds with the resin during its polymerization to produce a chemical bridge between the glass surface and the polymer.
The film former assists in incorporating the coupling agents into the sizing composition. The film former is a flexible constituent of the size that bonds the filaments together in a unit to give the filaments increased abrasion resistance during processing and handling. The best film formers available for the size composition are those that allow maximum adhesion between the glass fiber and the polymeric material by allowing the coupling agent to be in intimate contact with the glass fiber surface and the polymeric material. These types of film-formers are those that are soluble or compatible with the polymeric material thereby resulting in better wet-out of the fibers with the resinous material. A myriad of sundry film-formers having these properties have been developed by the art; for example, film-formers having polyester resin systems ranging from only the polyester resin to the polyester resin in addition with plasticizers and thermoplastic resins. The art has also recognized that when the polyester resin system has pendant carboxyl groups, such as those produced by the polymerization product of trimellitic anhydride or acid with an alcohol or glycol, the properties of the glass fiber reinforced polymer are improved. In addition, it has been suggested by the art that the bonding relationship between the resinous material and the glass fibers can be improved by having a sizing composition containing a film-former reaction product of a polyester resin film former with ethylenic unsaturation and a mercapto silane.
Typically the sizing composition is applied to the glass fibers during formation in a sizing solution having a total solids content of about 2 to 20 percent by weight. The total solids content of some polyester systems such as those containing a plasticizer have been reported to be as high as 23 percent by weight, but the usual total solids content of the sizing solution is around 5 to 17 percent by weight.
The quantity of sizing deposited on the glass fibers when a water soluble size is used, usually varies in loss on ignition of the strand between 0.6 percent on the interior of the forming package to 3.5 percent on the exterior of the forming package. The loss on ignition is a method of determining the quantity of size deposited on the glass fiber and involves taking a sample of glass fiber strand approximately around 11 meters, drying it at 110.degree. C. to constant weight, then burning off the sizing at a temperature of around 620.degree. to 640.degree. C.; the change in weight gives the quantity of sizing deposited on the glass fiber strand.
To produce a glass fiber strand with the desired quantity of sizing deposited on the strand by (LOI) loss on ignition, the total solids content of the sizing composition should not be too low. If the solids content of the sizing composition is too low, the water content of the formed glass fiber strand package will be too high and will require longer times in the drying process. In a forming package the water is usually concentrated in the interior of the package, and the weight of the forming package minus any structural winding element should be such that the layer of glass fibers is limited to a thickness of not more than 25 millimeters. When the glass fibers are to be used in the reinforcement of polymers, water can act as a plasticizer of the film-former, and impart handling characteristics to the strand which can be unsuitable. An example of such an unsuitable characteristic is where a fiber which is expected to be springy may turn out to be too limp if there is too much water present. In addition having too low of a solids content of a sizing leads to several disadvantageous results. For example, the efficiency of the use of the sizing composition is reduced, since the use of a sizing composition containing low solids and more water will lead to loss of the sizing composition during the forming and winding of the glass fiber strand due to the throw-off of the sizing composition during these operations. In addition when the water content of the sizing composition is high there is an increased opportunity for migration of the sizing composition from the inside to the outside of the forming package during drying. A further disadvantage of having a sizing composition with too high a water content is that it leads to longer drying times or higher drying or curing temperatures. Furthermore the use of a sizing composition having a high water content can be sloppy to work with and more difficult to clean up.
When a polyester resin system containing pendant carboxyl groups is used in the sizing composition, care must be exercised not to have too high a solids content in the sizing composition. If too high of a solids content is used, the size has a tendency to be unstable and to have material come out of solution.
It is an object of the present invention to provide a sizing composition which leads to higher sizing composition use efficiencies for the sizing composition by reducing the amount of throw-off during the forming and winding and gathering operations in the production of glass fiber strands.
It is an additional object of the present invention to provide a sizing composition for treating glass fibers that reduces the opportunity for migration in forming packages during drying.
It is an another object of the present invention to provide a sizing composition for treating glass fibers that requires lower or reduced drying or curing temperatures and time because of a reduced moisture content.
It is a further object of the present invention to provide a glass fiber sizing composition that improves housekeeping during formation of the glass fiber strands and lessens the time required for cleaning up the area around where the glass fiber strands are produced.
It is another additional object of the present invention to provide a sizing composition for treating glass fibers that has the capability of yielding treated glass fibers having LOIs greater than 3.5 percent.
It is a further additional object of the present invention to provide a sizing composition for treating glass fibers and a method of producing the sizing composition with good stability for original use and for recirculated use as a sizing composition for treating glass fibers.